1. Field of the Invention
The invention relates to a semiconductor device, and more particularly to a semiconductor device including interlayer insulative layers composed of insulative material having a low dielectric constant to thereby reduce wire delay and hence make it possible for a semiconductor device to operate at higher speed than a conventional semiconductor device. The invention also relates to a method of fabricating such a semiconductor device.
2. Description of the Related Art
With the demand of decreasing wiring width and a spacing between wirings in a semiconductor device, wire floating capacitance and wire resistance in a semiconductor device are increased with the result of increase of wire delay which is an obstacle to higher speed operation of a semiconductor device. Thus, for the purpose of decreasing wire delay, it is presently attempted to improve insulative material to be used for multiple wiring layers. Since wire delay is in general in proportion to a root of a dielectric constant of the insulative material, wire delay can be decreased by composing multiple wiring layers of insulative material having a low dielectric constant. Interlayer insulative films are presently composed of SiO.sub.2 and the like which has a dielectric constant of about 4, and it is now desired to develop insulative material having a dielectric constant of 3 or smaller. For this purpose, an attempt is being made to provide insulative material having a low dielectric constant such as SiOF film composed of SiO.sub.2 containing fluorine (F) to thereby reduce a dielectric constant, and organic material such as polyimide having a smaller dielectric constant than that of inorganic material such as SiO.sub.2.
For instance, the inventor had published a paper on plasma fluorination of polyimide in 55th Meeting of Applied Physics Institution, No. 3. 21a-G-11, Sep. 19, 1994. The fluorinated polyimide has a dielectric constant of 3 or smaller. It should be noted that the applicant does not admit the article No. 3. 21a-G-11 as prior art. This article is cited herein solely for better understanding of the background of the invention.
For another instance, Japanese Unexamined Patent Public Disclosure No. 4-174912 laid open on Jun. 23, 1992 has suggested a cable comprising a linear conductor having a diameter of 1 mm or less, and a plasma polymerized insulative film covering around the conductor, which film has a dielectric constant of 3 or smaller.
Still another instance is an article entitled "Mechanisms of etching and polymerization in radiofrequency discharges of CF.sub.4 --H.sub.2, CF.sub.4 --C.sub.2 F.sub.4, C.sub.2 F.sub.6 --H.sub.2, C.sub.3 F.sub.8 --H.sub.2 " reported by R. d'Agostino, F. Cramarossa, V. Colaprico, and R. d'Ettole through American Institute of Physics in J. Appl. Phys. 54(3), pp 1284-1288, March 1983. This report has reported some results obtained during the etching of Si or the deposition of fluorocarbon films over Si substrates uncoupled from ground in rf plasmas fed with CF.sub.4 --H.sub.2, C.sub.2 F.sub.6 --H.sub.2, C.sub.3 F.sub.8 --H.sub.2 and CF.sub.4 --C.sub.2 F.sub.4 mixtures.
Yet another instance is an article entitled "Electrical and Structural Studies of Plasma-polymerized Fluorocarbon Films" reported by N. Amyot, J. E. Klemberg-Sapieha, and M. R. Wertheimer in IEEE Transactions on Electrical Insulation, Vol. 27 No. 6, pp 1101-1107, December 1992. In this study, plasma-polymerized fluorocarbon films up to 8 .mu.m in thickness have been prepared by high frequency glow discharge deposition to investigate the material's charge storage (electret) properties. Under `mild` plasma conditions, materials with high fluorine concentration (F/C&lt;1.9) could be obtained, while films with lower F/C were found to be partially oxidized.
Still yet another instance is an article entitled "Plasma-deposited amorphous carbon films as planarization layers" reported by S. W. Pang and M. W. Horn through American Vacuum Society in J. Vac. Sci. Technol. B8(6), pp 1980-1984, November/December 1990. According to the report, a dry planarization process was developed that utilizes plasma-enhanced chemical vapor deposition of amorphous carbon films. The characteristics of the films depend on deposition conditions such as source gas composition, rf power, degree of ion bombardment, temperature, pressure, and electrode spacing. Planar films were deposited at low temperatures (&lt;50.degree. C.) with low ion bombardment energy (&lt;10 V) and high deposition rates (100-300 nm/min).
However, SiO.sub.2 containing fluorine therein does not exhibit sufficient decrease of a dielectric constant, and merely exhibits a dielectric constant of about 3. In addition, an interlayer insulative film composed of SiO.sub.2 containing fluorine has a problem with respect to hygroscopic property thereof. On the other hand, an interlayer insulative film composed of the polyimide resin also has problems that such a film has a low upper limitation with respect to heat resistance, specifically, the heat resistance of the film is just about 400 degrees centigrade, and that humidity present in the film exerts a bad influence on a semiconductor device in wet processes, and further that volumetric shrinkage which occurs while the polyimide resin is being cured may causes the film to be cracked.